Tightened emission control has brought about a strong demand for higher response to engine low velocity, and thus higher response of a turbocharger has been called for.
When the engine is accelerated, exhaust-gas pressure increases to provide energy to a turbine, and then the number of revolutions of the turbine increases. Thus, there is a problem that the number of revolutions of the turbine cannot be increased quickly due to a low turbine efficiency attributable to a turbine operational velocity ratio (U/C0) being much lower than a design point because of the early rise in the ratio of the pressure at the inlet of the turbine blade to the pressure at the outlet thereof.
Downsizing of the turbine and flow inclination to achieve lower inertia moment, which are currently conducted attempts to quickly increase the number of revolutions, ironically lead to further degradation of the turbine operational velocity ratio (U/C0) and performance degradation, and thus fail provide a sufficient response improvement effect.
FIG. 10A shows an example of a variable geometry turbocharger 3 using a radial turbine 1. In the figure, a spiral scroll 7 is formed in a turbine casing 5, and a gas outlet path 9 is formed on an inner circumference side. Furthermore, a compressor casing accommodating an unillustrated compressor, the turbine casing 5, and a bearing housing 11 are formed.
A turbine rotor 13 includes: a hub 17 fixed to an end portion of a rotor shaft 15; and a plurality of turbine blades 19 that fixedly adhere to an outer circumference of the hub 17 at an equal interval in a circumferential direction. The unillustrated compressor is coupled to a side of the rotor shaft 15 opposite to the turbine rotor 13.
A bearing 21 that supports the rotor shaft 15 is disposed in the bearing housing 11. The rotor shaft 15 and the hub 17 rotate about a center line of rotation 23.
A plurality of nozzle vanes 25 are disposed on an inner circumference side of the scroll 7 at an equal interval along the circumferential direction of the turbine rotor 13. The vane angle of the nozzle vane 25 can be changed by a variable nozzle mechanism 27.
While a variable geometry turbocharger 3 with the variable nozzle mechanism including the radial turbine 1 is under operation, exhaust gas from an internal combustion engine (not shown) enters the scroll 7 and flows into a space between the nozzle vanes 25 while swirling along the spiral in the scroll 7.
Thus, the exhaust gas flows through the space between the blades of the nozzle vanes 25, to flow from an inlet end surface on an outer circumference side of a plurality of turbine blades 19 into a space between the turbine blades 19. Then, the exhaust gas flows in the radial direction toward the center of the turbine rotor 13, to provide an expansion effect for the turbine rotor 13. Thereafter, the exhaust gas flows out in an axial direction to be emitted outside from the device through the gas outlet path 9.
In the variable geometry turbocharger 3, when the engine is accelerated, the nozzle vanes 25 are throttled, whereby a flowrate is reduced and a flow velocity is increased.
FIG. 7 and FIG. 8 show a velocity triangle formed by a circumferential direction velocity U at the leading edge 20 of the turbine blade 19, an absolute flowing-in velocity C, and a relative flowing-in velocity W. FIG. 7 shows a state of low U/C0, and FIG. shows a state of peak efficiency U/C0. As illustrated in FIG. 8 as an ideal case, the exhaust gas flows in from a slightly back side (negative pressure surface side 29) of the turbine blade 19.
However, when the nozzle vanes 25 are strongly throttled, the low U/C0 state shown in FIG. 7 is achieved where the absolute flow velocity is excessively inclined (dotted line in FIG. 6). Thus, the exhaust gas flows in from the front side (pressure surface side) 31 of the turbine blade.
As described above, a reduction of the turbine operational velocity ratio U/C0 leads to a large reduction of a flow angle α at the leading edge. Thus, the flowing exhaust gas collides with the pressure surface side 31 due to a large shift between a leading edge angle (metal angle) β of the turbine blade 19 and an angle of the flow. Thus, a leakage flow of the exhaust gas flowing around to reach the negative pressure surface side 29 from the pressure surface side 31, and an excessively large separated flow toward the negative pressure surface side are produced to cause an impact loss and efficiency degradation. FIG. 9 shows a state of an impact loss produced on the negative pressure surface side 29 in the state of low U/C0.
Patent Document 1 (Japanese Patent Application Laid-open No. 2011-132810) discloses an example of making a leading edge angle of the radial turbine blade match a direction of a flow of inflowing gas. More specifically, Patent Document 1 discloses a configuration in which a blade distal end shape adjacent to both walls on a shroud side 056 and a hub side 054 defining a height direction of an inlet 052 of a turbine blade 050 into which working gas flows has a direction changed to match a flowing-in direction of a gas relative flowing-in velocity component as shown in FIG. 11A and FIG. 11B.